What you do is create a script that you place on top of every page that only runs every 5 minutes. It won't be guaranteed to run every 5 minutes though. As it requires someone to visit the pages with the "poor man's cron" and if no one visits for over 5 minutes then it won't run until someone does.

Some example code:

<?php
// load the last run time from a file, database, etc
if(time() >= $last_run + (60 * 5)) { // 60 * 5 is 5 minutes
// do your task here
// save the last run time to a file, database, etc
}

Here's another 'hack'. Since you can't run cron on the machine where the script is, maybe you can run cron on another machine.

If you can...setup a cron job to run every 5 minutes...The job can be a simple PHP script that calls your other PHP script. You can use cURL to 'call' your script (if that script is being served up by a Web Server)

PHP Cron Daemon is driven by a database to schedule the execution of task (like cron). It can parse a crontab file and extract the job scheduling definitions into a MySQL database table. When it is time to run a scheduled job, it executes the job command in the crontab definition as PHP code as a separate process. The output of the job PHP code is also stored in a database table.